The present invention relates generally to circuit board assembly and more particularly to an improved method and apparatus for mounting electronic components to printed circuit boards.
Most electronic devices today include a large number of electronic components or integrated circuits (ICs) which are mounted to circuit boards which are arranged within the device. The components are attached to the boards and each of the components has a large number of electrical leads which must be individually connected to specific electrical points on the circuit board. The components are held to the board with adhesive materials and also, at least in part, by the soldered electrical connections.
In order to increase the number of components than can be mounted within a given device volume and therefore decrease the size of, or miniaturize, many electronic devices, circuit boards are being assembled having components mounted on both sides of a circuit board instead of having components mounted on only one side of the board. During the assembly process, circuit board components are mounted on a first side of the circuit board during a first pass-through of a soldering oven during which a mixture of solder beads and flux between the components and the circuit board are heated to a point at which the solder begins to flow as a liquid. As the circuit board completes the oven process, the solder cools and hardens thereby forming a solid electrical and mechanical connection between the component connection points and the corresponding connection points on the circuit board. The components are mounted on the second side of the circuit board by inverting the board and running a second pass of the board through the soldering oven during which components are mounted on the second side of the circuit board.
During the second pass through the solder oven, the components which were mounted during the first pass are on the bottom side of the board and are subject to the force of gravity which asserts a force tending to pull the components downwardly and away from the board. As the first side components pass the xe2x80x9creflowxe2x80x9d area of the solder oven, the soldered connections made during the first pass change state from solid to liquid or xe2x80x9creflowxe2x80x9d and reduce the surface tension which holds the components to the circuit boards. When the ratio of the mass of the components to the surface tension holding the components to the boards exceeds a certain value, the first pass soldered connections will become weak and there is a tendency for the components mounted during the first pass to fall off the underside of the board and the board will have to be reworked.
The industry""s current emphasis on miniaturization and environmentally-friendly soldering (inert gas blanket, no-clean flux and low solid flux) increases the component retention problem because the trends are to reduce the surface area and the surface tension of the solder. While some new techniques aid the soldering process during a first pass, such techniques also increase the fall-off rate for components going through a second pass on the underside of a circuit board.
Thus, there is a need for an improved methodology and implementing system for mounting electronic components on printed circuit boards and controlling solder reflow.
A method and implementing system is provided in which a gas is injected on to an area of a circuit board to which an electronic component is being mounted. The injected gas causes a formation of a coating or surface alloy layer on the solder bead. The coating causes the solder bead to have a higher surface tension and a higher reflow temperature. In an exemplary double-sided double-pass assembly operation, circuit boards pass through a soldering oven on a first pass to attach components to a first side, and then the board is inverted and passed through the oven on a second pass while components are mounted on the opposite side of the board. During the second pass, a gas injection device is aimed at the component-to-board connection points on the inverted side of the board which were soldered on the first pass. The gas is injected at the point in the soldering reflow oven at which the temperature begins to exceed the solder reflow temperature. The gas injection is effective to cause the formation of a surface layer on the solder bead thereby increasing the surface tension of the molten solder between the circuit board and the mounted component during the second pass reflow. The temperature of the injected gas may also be varied to achieve desirable results.